Ataxin-1 (Atx1) is a neurodegenerative disorder protein whose expanded glutamine-repeat form causes spinocerebellar ataxia type 1 (SCA1) in humans and which exerts cytotoxicity in mice and in Drosophila. Although the linkage between Atx1 and SCA1 has been established for more than a decade, the exact properties of Atx1 and their connection to the pathogenesis of SCA1 remain unclear. It was long thought that the expanded glutamine-repeat in Atx1 is the main culprit that brings about toxic effects, yet growing evidence indicates that the protein's inherent properties also play an important role in SCA1 pathology. In our research, we recently established that Atx1 binds directly to a class of conserved transcriptional co- repressors, including vertebrate SMRT (Silencing Mediator of Retinoid and Thyroid hormone receptors) and its Drosophila cognate SMRTER, both in vitro and in vivo. Consequently, we hypothesize that one key aspect of AtxVs properties involves transcriptional regulation, and that perturbation of conserved SMRT/SMRTER-dependent regulatory pathways is partly responsible for its cytotoxicity in both vertebrates and invertebrates. Because both SMRT and SMRTER are nuclear receptor co-repressors, this proposal probes whether Atx1-mediated cytotoxicity involves SMRT/SMRTER and their associating nuclear receptors. In Specific Aim 1, we propose various molecular biology approaches, including the chromatin immunoprecipitation method, to investigate whether Atx1, by means of its interaction with SMRT, is involved in thyroid hormone receptor (TR) signaling in vertebrates. In Specific Aim 2, we will use genetic approaches to elucidate the relationships among Atx1, SMRTER, and ecdysone receptor (the functional equivalent of TR) in Drosophila. Finally, in Specific Aim 3, building on our recent discovery of two functional domains of Atx1, the SMRT/SMRTER interacting AXH (Atx1 and HMG-box protein) domain and a self-association NBA (N-terminal region of Boat and Atx1) domain, we will investigate the specific consequences that these two domains may have for Atxl's cytotoxicity. Connecting the functions of Atx1 to nuclear hormone receptor signaling will have major implications, since it suggests that intervention with hormones or with chemical compounds that affect the operation of nuclear hormone receptors can be exploited to develop therapeutic agents for treating SCA1.